2Precompiler Concepts

This chapter explains how embedded SQL programs do their work. You examine the special environment in which they operate and the impact of this environment on the design of your applications. After covering the key concepts of embedded SQL programming and the steps you take in developing an application, this chapter uses a simple program to illustrate the main points.

2.1.1Embedded SQL Statements

The term embedded SQL refers to SQL statements placed within an application program. Because it houses the SQL statements, the application program is called a host program, and the language in which it is written is called the host language. For example, Pro*C/C++ provides the ability to embed certain SQL statements in a C or C++ host program.

The powerful SET ROLE statement lets you dynamically manage database privileges. A role is a named group of related system and object privileges, or a named group of related system or object privileges granted to users or other roles. Role definitions are stored in the Oracle data dictionary. Your applications can use the SET ROLE statement to enable and disable roles as needed.

2.1.1.1Executable Statements and Directives

Embedded SQL includes all the interactive SQL statements plus others that allow you to transfer data between Oracle and a host program. There are two types of embedded SQL statements: executable statements and directives. Executable statements result in calls to the runtime library SQLLIB. You use them to connect to Oracle, to define, query, and manipulate Oracle data, to control access to Oracle data, and to process transactions. They can be placed wherever C or C++ language executable statements can be placed.

Directives, on the other hand, do not result in calls to SQLLIB and do not operate on Oracle data. You use them to declare Oracle objects, communications areas, and SQL variables. They can be placed wherever C or C++ variable declarations can be placed.

2.1.2 Embedded SQL Syntax

In your application program, you can freely mix complete SQL statements with complete C statements and use C variables or structures in SQL statements. The only special requirement for building SQL statements into your host program is that you begin them with the keywords EXEC SQL and end them with a semicolon. Pro*C/C++ translates all EXEC SQL statements into calls to the runtime library SQLLIB.

Many embedded SQL statements differ from their interactive counterparts only through the addition of a new clause or the use of program variables. The following example compares interactive and embedded ROLLBACK statements:

ROLLBACK WORK: -- interactive
EXEC SQL ROLLBACK WORK; -- embedded

These statements have the same effect, but you would use the first in an interactive SQL environment (such as when running SQL*Plus), and the second in a Pro*C/C++ program.

2.1.3 Static Versus Dynamic SQL Statements

Most application programs are designed to process static SQL statements and fixed transactions. In this case, you know the makeup of each SQL statement and transaction before runtime; that is, you know which SQL commands will be issued, which database tables might be changed, which columns will be updated, and so on.

However, some applications might be required to accept and process any valid SQL statement at runtime. So, you might not know until runtime all the SQL commands, database tables, and columns involved.

Dynamic SQL is an advanced programming technique that lets your program accept or build SQL statements at run time and take explicit control over datatype conversion.

2.1.4 Embedded PL/SQL Blocks

Pro*C/C++ treats a PL/SQL block like a single embedded SQL statement. You can place a PL/SQL block anywhere in an application program that you can place a SQL statement. To embed PL/SQL in your host program, you simply declare the variables to be shared with PL/SQL and bracket the PL/SQL block with the keywords EXEC SQL EXECUTE and END-EXEC.

From embedded PL/SQL blocks, you can manipulate Oracle data flexibly and safely because PL/SQL supports all SQL data manipulation and transaction processing commands.

2.1.5 Host and Indicator Variables

Host variables are the key to communication between Oracle and your program. A host variable is a scalar or aggregate variable declared in C and shared with Oracle, meaning that both your program and Oracle can reference its value.

Host variables can be used anywhere a SQL expression can be used. In SQL statements, host variables must be prefixed with a colon (":") to set them apart from the SQL keywords.

You can also use a C struct to contain a number of host variables. When you name the structure in an embedded SQL statement, prefixed with a colon, Oracle uses each of the components of the struct as a host variable.

You can associate any host variable with an optional indicator variable. An indicator variable is a short integer variable that "indicates" the value or condition of its host variable. You use indicator variables to assign NULLs to input host variables and to detect NULLs or truncated values in output host variables. A NULL is a missing, unknown, or inapplicable value.

In SQL statements, an indicator variable must be prefixed with a colon and immediately follow its associated host variable. The keyword INDICATOR can be placed between the host variable and its indicator for additional clarity.

If the host variables are packaged in a struct, and you want to use indicator variables, you simply create a struct that has an indicator variable for each host variable in the host structure, and name the indicator struct in the SQL statement, immediately following the host variable struct, and prefixed with a colon. You can also use the INDICATOR keyword to separate a host structure and its associated indicator structure.

2.1.6 Oracle Datatypes

Typically, a host program inputs data to Oracle, and Oracle outputs data to the program. Oracle stores input data in database tables and stores output data in program host variables. To store a data item, Oracle must know its datatype, which specifies a storage format and valid range of values.

Oracle recognizes two kinds of datatypes: internal and external. Internal datatypes specify how Oracle stores data in database columns. Oracle also uses internal datatypes to represent database pseudocolumns, which return specific data items but are not actual columns in a table.

External datatypes specify how data is stored in host variables. When your host program inputs data to Oracle, if necessary, Oracle converts between the external datatype of the input host variable and the internal datatype of the target database column. When Oracle outputs data to your host program, if necessary, Oracle converts between the internal datatype of the source database column and the external datatype of the output host variable.

2.1.7 Arrays

Pro*C/C++ lets you define array host variables (called host arrays) and arrays of structures and operate on them with a single SQL statement. Using the array SELECT, FETCH, DELETE, INSERT, and UPDATE statements, you can query and manipulate large volumes of data with ease. You can also use host arrays inside a host variable struct.

2.1.8 Datatype Equivalencing

Pro*C/C++ adds flexibility to your applications by letting you equivalence datatypes. That means you can customize the way Oracle interprets input data and formats output data.

On a variable-by-variable basis, you can equivalence supported C datatypes to the Oracle external datatypes. You can also equivalence user-defined datatypes to Oracle external datatypes.

2.1.9 Private SQL Areas, Cursors, and Active Sets

To process a SQL statement, Oracle opens a work area called a private SQL area. The private SQL area stores information needed to execute the SQL statement. An identifier called a cursor lets you name a SQL statement, access the information in its private SQL area, and, to some extent, control its processing.

For static SQL statements, there are two types of cursors: implicit and explicit. Oracle implicitly declares a cursor for all data definition and data manipulation statements, including SELECT statements (queries) that return only one row. However, for queries that return more than one row, to process beyond the first row, you must explicitly declare a cursor (or use host arrays).

The set of rows returned is called the active set; its size depends on how many rows meet the query search condition. You use an explicit cursor to identify the row currently being processed, called the current row.

Imagine the set of rows being returned to a terminal screen. A screen cursor can point to the first row to be processed, then the next row, and so on. In the same way, an explicit cursor "points" to the current row in the active set. This allows your program to process the rows one at a time.

2.1.10 Transactions

A transaction is a series of logically related SQL statements (two UPDATEs that credit one bank account and debit another, for example) that Oracle treats as a unit, so that all changes brought about by the statements are made permanent or undone at the same time.

All the data manipulation statements executed since the last data definition, COMMIT, or ROLLBACK statement was executed make up the current transaction.

To help ensure the consistency of your database, Pro*C/C++ lets you define transactions using the COMMIT, ROLLBACK, and SAVEPOINT statements.

COMMIT makes permanent any changes made during the current transaction. ROLLBACK ends the current transaction and undoes any changes made since the transaction began. SAVEPOINT marks the current point in the processing of a transaction; used with ROLLBACK, it undoes part of a transaction.

2.1.11 Errors and Warnings

When you execute an embedded SQL statement, it either succeeds or fails, and might result in an error or warning. You need a way to handle these results. Pro*C/C++ provides two error handling mechanisms: the SQL Communications Area (SQLCA) and the WHENEVER statement.

The SQLCA is a data structure that you include (or hard-code) in your host program. It defines program variables used by Oracle to pass runtime status information to the program. With the SQLCA, you can take different actions based on feedback from Oracle about work just attempted. For example, you can check to see if a DELETE statement succeeded and, if so, how many rows were deleted.

With the WHENEVER statement, you can specify actions to be taken automatically when Oracle detects an error or warning condition. These actions are: continuing with the next statement, calling a function, branching to a labeled statement, or stopping.

2.2 Steps in Developing an Embedded SQL Application

As you can see, precompiling results in a modified source file that can be compiled normally. Though precompiling adds a step to the traditional development process, that step lets you write very flexible applications.

2.3 Guidelines for Programming

This section deals with embedded SQL syntax, coding conventions, and C-specific features and restrictions. Topics are arranged alphabetically for quick reference.

2.3.1 Comments

You can place C-style Comments (/* ... */) in a SQL statement wherever blanks can be placed (except between the keywords EXEC SQL). Also, you can place ANSI-style Comments (-- ...) within SQL statements at the end of a line, as the following example shows:

You can use C++ style Comments (//) in your Pro*C/C++ source if you precompile using the CODE=CPP precompiler option.

2.3.2 Constants

An L or l suffix specifies a long integer constant, a U or u suffix specifies an unsigned integer constant, a 0X or 0x prefix specifies a hexadecimal integer constant, and an F or f suffix specifies a float floating-point constant. These forms are not allowed in SQL statements.

2.3.3 Declare Section

A Declare Section contains the host variable declarations and is of the form:

SQL uses double quotes to delimit identifiers containing special or lowercase characters, as in

EXEC SQL CREATE TABLE "Emp2" (empno number(4), ...);

2.3.5 File Length

Pro*C/C++ cannot process arbitrarily long source files. There is a limit to the number of lines allowed. The following aspects of the source file are contributing factors to the file-size constraint:

Complexity of the embedded SQL statements (for example, the number of bind and define variables).

Whether a database name is used (for example, connecting to a database with an AT clause).

Number of embedded SQL statements.

To prevent problems related to this limitation, use multiple program units to sufficiently reduce the size of the source files.

2.3.6 Function Prototyping

The ANSI C standard (X3.159-1989) provides for function prototyping. A function prototype declares a function and the datatypes of its arguments, so that the C compiler can detect missing or mismatched arguments.

The CODE option, which you can enter on the command line or in a configuration file, determines the way that the precompiler generates C or C++ code.

2.3.6.1 ANSI_C

When you precompile your program with CODE=ANSI_C, the precompiler generates fully prototyped function declarations. For example:

extern void sqlora(long *, void *);

2.3.6.2 KR_C

When you precompile with the option CODE=KR_C (KR for "Kernighan and Ritchie"), the precompiler generates function prototypes in the same way that it does for ANSI_C, except that function parameter lists are commented out. For example:

extern void sqlora(/*_ long *, void * _*/);

So, make sure to set the precompiler option CODE to KR_C if you use a C compiler that does not support ANSI C. When the CODE option is set to ANSI_C, the precompiler can also generate other ANSI-specific constructs; for example, the const type qualifier.

2.3.6.3 CPP

When you compile with CODE=CPP you will generate C++ compatible function prototypes. Use this option setting with C++ compilers.

2.3.7 Host Variable Names

Host variable names can consist of upper or lowercase letters, digits, and underscores, but must begin with a letter. They can be any length, but only the first 31 characters are significant to Pro*C/C++. Your C compiler or linker might require a shorter maximum length, so check your C compiler user's guide.

For SQL92 standards conformance, restrict the length of host variable names to 18 or fewer characters.

2.3.11 Operators

The logical operators and the "equal to" relational operator are different in C and SQL, as the following list shows. These C operators are not allowed in SQL statements:

SQL Operator

C Operator

NOT

!

AND

&&

OR

||

=

==

The following C operators also not allowed in SQL statements:

Type

C Operator

address

&

bitwise

&, |, ^, ~

compound assignment

+=, -=, *=, and so on.

conditional

?:

decrement

--

increment

++

indirection

*

modulus

%

shift

>>,<<

2.3.12 Statement Terminator

Embedded SQL statements are always terminated by a semicolon, as the following example shows:

EXEC SQL DELETE FROM emp WHERE deptno = :dept_number;

2.4 Conditional Precompilation

Conditional precompilation includes (or excludes) sections of code in your host program based on certain conditions. For example, you might want to include one section of code when precompiling under UNIX and another section when precompiling under VMS. Conditional precompilation lets you write programs that can run in different environments.

Conditional sections of code are marked by statements that define the environment and actions to take. You can code C or C++ statements as well as EXEC SQL statements in these sections. The following statements let you exercise conditional control over precompilation:

You can "comment out" C, C++, or embedded SQL code by placing it between IFDEF and ENDIF and not defining the symbol.

2.5 Precompile Separately

You can precompile several C or C++ program modules separately, then link them into one executable program. This supports modular programming, which is required when the functional components of a program are written and debugged by different programmers. The individual program modules need not be written in the same language.

2.5.1 Guidelines

The following guidelines will help you avoid some common problems.

2.5.1.1 Referencing Cursors

Cursor names are SQL identifiers, whose scope is the precompilation unit. Hence, cursor operations cannot span precompilation units (files). That is, you cannot DECLARE a cursor in one file, and OPEN or FETCH from it in another file. So, when doing a separate precompilation, make sure all definitions and references to a given cursor are in one file.

2.5.1.2 Specifying MAXOPENCURSORS

When you precompile the program module that CONNECTs to Oracle, specify a value for MAXOPENCURSORS that is high enough for any of the program modules. If you use MAXOPENCURSORS for another program module, one that does not do a CONNECT, then that value for MAXOPENCURSORS is ignored. Only the value in effect for the CONNECT is used at runtime.

2.5.1.3 Use a Single SQLCA

If you want to use just one SQLCA, you must declare it as global in one of the program modules and as external in the other modules. Use the extern storage class, and the following define in your code:

#define SQLCA_STORAGE_CLASS extern

which tells the precompiler to look for the SQLCA in another program module. Unless you declare the SQLCA as external, each program module uses its own local SQLCA.

Note:

All source files in an application must be uniquely named, or else an error will be generated.

2.6 Compile and Link

To get an executable program, you must compile the output .c source files produced by the precompiler, then link the resulting object modules with modules needed from SQLLIB and system-specific Oracle libraries. If you are mixing precompiler code and OCI calls, be sure to also link in the OCI runtime library (liboci.a on UNIX systems).

The linker resolves symbolic references in the object modules. If these references conflict, the link fails. This can happen when you try to link third-party software into a precompiled program. Not all third-party software is compatible with Oracle. So, linking your program shared might cause an obscure problem. In some cases, linking standalone or two-task might solve the problem.

Compiling and linking are system dependent. On most platforms, example makefiles or batch files are supplied that you can use to precompile, compile, and link a Pro*C/C++ application. See your system-specific documentation.

2.7 Example Tables

Most programming examples in this guide use two example database tables: DEPT and EMP. Their definitions follow:

2.8 Example Program: A Simple Query

One way to get acquainted with Pro*C/C++ and embedded SQL is to study a program example. The following program is also available on-line in the file sample1.pc in your Pro*C/C++ demo directory.

The program connects to Oracle, then loops, prompting the user for an employee number. It queries the database for the employee's name, salary, and commission, displays the information, and then continues the loop. The information is returned to a host structure. There is also a parallel indicator structure to signal whether any of the output values SELECTed might be NULL.